TMAs have been recently gaining relevance in the cellular network industry. Basically, a TMA is a device which comprises a reception and transmission filter and a low noise amplifier (LNA). Such devices, usually installed directly behind the base station antennas, amplify the received signals at the top of the base station antenna mast so as to improve the overall system sensibility of a mobile network base station. By amplifying the signals as near as possible to the antenna, the TMA compensates the cable and connector losses between the antenna and the input of the base station.
In a TDD communication system, uplink and downlink transmissions use a same frequency. Therefore, in a typical TMA equipment, for example as described in a U.S. patent application Ser. No. 10/976,841, there may be a risk for the LNA included in the TMA to withstand a high power signal reflected from an antenna port due to the mismatch of the antenna port. The reflected energy is usually proportional to the degree of the mismatch. The more the antenna port is mismatched, the more energy may be reflected from the antenna port to the LNA. Especially, when the antenna port is open, all transmitting energy may be reflected to the LNA, which is vital and may cause the LNA to be broken. Therefore, protection for the LNA in a TMA is critical in a TDD communication system.
An existing solution as shown in FIG. 1 employs circulators to separate receiving (Rx) and transmitting (Tx) signals. In order to protect the LNA from damaging by the possibly returned energy during a Tx slot, a single pole double throw (SPDT) switch is used before it.
However, there exist some disadvantages with the existing solution. For example, since insertion loss (IL) of a high power TR (Tx-Rx) switch, such as the SPDT switch, is generally above 0.5 dB and the IL of a circulator is typically 0.3 dB, the total IL except for the IL introduced by an RF filter on the Rx path is up to 1.1 dB. Additionally, the dimension of a TR switch is large, for example, about 45 mm×5 mm for a 40 W system, which may result in a bulky TMA.